1. Field of the Invention
This invention relates to an improved system for controlling the lock-up clutch of torque converter of a vehicle automatic transmission system.
2. Description of the Prior Art
In conventional control of the lock-up clutch of an automatic transmission used in a vehicle, the engagement range of the lock-up clutch is determined by retrieval from a two-dimensional shift diagram stored in a microcomputer memory as a map, hereinafter referred to as "shift diagram map", using the vehicle speed and the throttle opening as parameters. However, it is not possible to determine the motive force (reserve engine power) of the vehicle solely from these two parameters, as is obvious from the fact that a throttle opening and vehicle speed appropriate for level-road driving will not provide adequate motive force during hill-climbing. On the other hand, from the points of fuel economy, noise suppression and the like it is necessary to expand the engagement range of the lock-up clutch so as to reduce torque converter slip and thus increase the power transmission efficiency and reduce the engine speed. It is extremely difficult to overcome both problems with control based on the conventional shift diagram map. If, for example, the engagement range is expanded for optimum performance during level-road driving, the drivability will be markedly degraded by insufficient motive force during hill-climbing. However, if an attempt is made to secure good drivability over a wide range of driving conditions including hill-climbing, the engagement range of the lock-up clutch will become so narrow as to impair its effectiveness during high-speed driving and deceleration.
Moreover, when the accelerator pedal of a vehicle equipped with an automatic transmission controlled in the conventional manner using the shift diagram map is frequently depressed and released, as happens during driving in a mountainous area or as the driver may do at his or her own volition, the lock-up clutch frequently engages and disengages, with the result that clutch wear increases and the fuel economy is degraded rather than enhanced. Briefly stated, what this means is that the prior art map-based lock-up clutch control is incapable of responding to the intentions and expectations of the driver, who tends to use his or her impressions of the operations of an expert driver driving a vehicle with a manual transmission as a standard for evaluating the control. Thus for providing control more closely matched to the intentions, judgments and feelings of the driver, Japanese Laid-open Patent Publication No. 3(1991)-103665 proposes a control system in which the road grade is determined from the vehicle speed and the engine output, and fuzzy reasoning is conducted on the basis of engine and driving parameters including the road grade for controlling engagement/disengagement of the lock-up clutch.
However, since this prior art control system only provides ON/OFF control between the engaged and disengaged states of the lock-up clutch in the third and fourth gears of the D range and does not provide control for semi-engaged clutch states, it does not take full advantage of the capability of the lock-up clutch to improve torque converter transmission efficiency while simultaneously using clutch semi-engagement to suppress the engine vibration that tends to become a problem during low speed driving.
In addition, there are two types of fuzzy reasoning, that using fuzzy relationships and that using fuzzy production rules, and this prior art system controls clutch engagement/disengagement by fuzzy reasoning using fuzzy relationships. Specifically, there are established rules whose antecedents specify the conditions to be met for engagement/disengagement of the lock-up clutch, the degree of satisfaction of each control rule is determined and compared with that of the other rules, and the consequent of the rule whose antecedent has the greatest degree of satisfaction is selected for implementation. Reasoning by fuzzy relationships deals with a model expressed as complex relationships involving various fuzzy relationships connected in series and parallel. This form of reasoning is suitable for troubleshooting or diagnosis and other types of backward reasoning but is not appropriate for forward reasoning which, as in the case of decisions regarding lock-up, requires control values to be determined through the analysis of current phenomena. In contrast, fuzzy reasoning by rules uses a model expressed on the control side in terms of "If . . . , then . . . ," permit human behavior patterns to be easily expressed and the creation of a knowledge base to be facilitated. As this mode of reasoning also makes it possible to formulate rules in the manner of a dialogue, it enables the control know-how acquired through the experience of an expert driver to be incorporated with increased accuracy so that control and response matched to human feelings can be more readily realized Moreover, modifications in the control rules are easy to implement.
The aforesaid prior art control determines the road grade through calculations without using a sensor. While this has the advantage of reducing the cost of the sensor system, it is disadvantageous in that the calculated value is only an estimate and is not necessarily adequate as regards reliability.